A mobile phone system generally comprises a base station which is connected with a communication network and a mobile station such as portable telephone or the like. In this system, a communicable distance between the base station and the mobile station is closely related to the transmittable power. Therefore, in order to cover a wide area, a method of transmitting same signals with same frequencies from a plurality of base stations is considered.
There will be concretely explained below a conventional radio communication system with reference to drawings. For example, FIG. 11 is a diagram showing a structure of the conventional radio communication system having a structure for covering a wide area. In FIG. 11, legends 107A and 107B represent base stations, legend 101 represents a transmission information input terminal into which transmission information is input from a network. Legends 102A and 102B represent modulators, legends 103A and 103B represent antennas of the base stations, legends 104 represents a mobile station, and legend 105 represents an antenna of the mobile station. Here, an internal structure of the base stations 107A and 107B will be described concentrating at the modulators 102A and 102B which have the most important function. The base station 107A transmits information via the antenna 103A. Similarly the base station 107B transmits information via the antenna 103B. The mobile station 104 receives transmitted signals from the two base stations 107A and 107B via the antenna 105.
In the radio communication system having the above structure, normally the radio wave environment is determined by a positional relationship between the mobile station 104 and the base stations 107A and 107B. FIG. 12 is a timing chart showing radio wave propagation in the conventional radio communication system. For example, the mobile station 104 receives the transmitted signals from the two base stations 107A and 107B simultaneously so that wide range coverage can be realized. However, as shown in FIG. 12, in a specified area (in the case where distances between the mobile station 104 and the two base stations are approximately equal), the power of the signal RA received from the base station 107A is equal to the power of the signal RB received from the base station 107B, but their polarities become occasionally opposite to each other. In this case, the two received signals RA and RB offsets each other, and when they are synthesized, no signal exists.
Another example of the conventional radio communication system is a radio communication system disclosed in Patent Gazette No. 2572765. For example, in this radio communication system, a base station is provided with a plurality of antennas, and a method of delaying transmission signals by not less than 1 symbol by means of a delay unit is used. As a result, the radio communication system which covers a wider area can be realized. FIG. 13 is a diagram showing a structure of such a radio communication system where a plurality of antennas are provided to the base station and which point is different from FIG. 11. In FIG. 13, legend 107 represents a base station, legend 101 represents a transmission information input terminal where transmission information is input from a network, and legend 102 represents a modulator. Further, legend 103A represents a first antenna of the base station 107, legend 106 represents a delay unit, legend 103C represents a second antenna of the base station 107, legend 104 represents a mobile station and legend 105 represents an antenna of the mobile station 104.
In the radio communication system having the above structure, the base station 107 transmits information via the first antenna 103A, and the delay unit 106 delays the same information by not less than 1 symbol. Thereafter, the base station 107 transmits the information via the second antenna 103C. The mobile station 104 receives signals transmitted from the two antennas 103A and 103C of the base station 107 via the antenna 105. At this time, since the signals from the two antennas on the transmission side have time difference of not less than 1 symbol, the time difference is corrected by an equalizer in the mobile station 104.
Further, in the radio communication system shown in FIG. 13, if the radio wave environments in the transmission antennas 103A and 103C are independent of each other, the phenomenon such that the received signals offset each other and no signal exists can be eliminated by a diversity effect. As a result, the characteristic can be improved. However, in this structure, the base station is only one, and the transmission signals are delayed by not less than 1 symbol. For this reason, a circuit size of the equalizer in the mobile station is disadvantageously increased, and thus this structure is insufficient to solve the problem.
That is, as shown in FIG. 11, when the radio communication system which covers a wide area is realized, there arises a problem that the signals from a plurality of base stations offset each other and reception of the signals is difficult in a specified position. On the other hand, as shown in FIG. 13, when the base station outputs a normal transmission signal and a transmission signal delayed from the normal transmission signal, there arises a problem that the structure of the equalizer on the receiving side becomes complicated.
The present invention is devised in order to solve the above problems. It is an object of the invention to provide a radio communication system, transmitter and receiver which previously prevent the phenomenon that all signals decay after synthesization in mobile stations between a plurality of base stations and are capable of covering a wide area with a simple structure.